1. Field Of the Invention
The present invention is directed to a retaining wall system. More particularly, the present invention is directed to a soil or rock nail retaining wall which includes a permanent outer face comprised of facing elements. A method of constructing such a retaining wall is also provided, including a method for connecting the soil or rock nails to the facing elements. The disclosed soil or rock nail wall and method are adapted for use with excavated cuts in many types of ground. The disclosed invention allows for the use of a permanent soil or rock nail wall in geographic locations where the ground freezes to significant depths and exerts pressure on the facing elements which apply additional loads to the soil or rock nails.
2. Description of the Prior Art
Tied Back Walls
One prior art method of supporting the sides of excavations is to use a tied back wall. A tied back wall utilizes a plurality of tiebacks. A tieback comprises a grouted anchor installed in the ground to secure a tendon which applies a force on the retaining wall. The tiebacks are anchored in the ground behind the wall and apply the force necessary to support the soil mass.
U.S. Pat. No. 4,561,804 shows one type of tied back wall. The exposed face of the soil is supported in part by vertically disposed sheet piles and either timber lagging or a layer of pneumatically applied concrete. The soil is then removed in descending stages until further support becomes necessary. At this point, tiebacks are installed through the sheet piles and into the ground. The tiebacks are then secured, tested and prestressed against the sheet piles. Excavation continues to the subgrade while lagging and, if required, more tiebacks are installed. A final layer of poured concrete is provided to form the finished, permanent retaining wall.
Other tied back wall systems are shown in U.S. Pat. Nos. 4,836,718 and 5,356,242. The disclosures of these patents provide a method for connecting soldier beams to precast panels. Both systems utilize a bulkhead or form system in conjunction with a cast-in-place concrete closure pour. However, both systems suffer from significant disadvantages. Most notably, both systems require the installation of vertical piles or soldier beams. Soldier beam installation is very costly and extremely difficult to accomplish in rock or rock-like ground formations. Further, neither system provides for the temporary support of the precast panels during construction of the wall. In addition, the configuration of U.S. Pat. No. 4,836,718 is somewhat disadvantageous because the bulkhead must be inflated. This system is thus extremely difficult to construct adjacent to an irregularly-shaped surface, such as an excavated cut face or an excavated cut face covered with a thin layer of pneumatically applied concrete.
Tied Back Element Walls (TE Wall)
A tied back element wall (TE wall) similarly uses a plurality of tiebacks. In a TE wall, the force necessary to support the side of an excavation is applied by prestressing the tiebacks against retaining elements disposed along the exposed face of the soil mass. The soil in front of the wall is excavated from the top down in successive sections. A layer or layers of pneumatically applied concrete is then applied to help support the exposed face of the excavated section of the soil. Retaining elements are next positioned along the exposed face of the soil or pneumatically applied concrete, and tiebacks are installed through the retaining elements and into the ground. Finally, the tiebacks are tested and prestressed against the retaining elements, and these steps repeated as needed until the entire wall is constructed.
Tied Back Element and Soil Nail Wall (TEN Wall)
A tied back element and soil nail wall (TEN wall) is a combination of a tied back element wall and a soil nail wall (soil nail walls are discussed in more detail below). A TEN wall is made up of a plurality of tiebacks, retaining wall elements and soil nails. This method utilizes short soil nails and pneumatically applied concrete to support a section of the soil to a certain excavated depth. At some point of the excavation, when further support becomes necessary to retain the soil, a row of tiebacks is added. By using the soil nails and pneumatically applied concrete as support between tiebacks, the wall uses both soil nails and tiebacks for support. In the TEN wall method, both the soil nails and the tiebacks form a part of the final retaining wall support structure. The ultimate strength of the retaining wall will thus depend on the strength of the soil nails and tiebacks themselves. An outer facing in addition to the pneumatically applied concrete is not used, and no further reinforcement is provided to the retaining wall structure.
A slightly different prior art retaining wall system is shown and described in U.S. Pat. No. 5,395,185. This system similarly utilizes soil nails, pneumatically applied concrete, and tiebacks, but also includes an outer face. The soil nails and pneumatically applied concrete are used to temporarily support the face of an excavated cut. Tiebacks are also installed, with their exposed ends extending a nominal distance from the face of the excavated cut. A permanent outer concrete facing is then poured and cured over the face of the excavated cut. After the concrete facing is applied, the tiebacks, via their exposed ends, are prestressed against the outer concrete facing. Because both tiebacks and soil nails must be used, this method is comparatively expensive. Also, with this method, the tiebacks either visibly protrude through the completed wall or the wall must be patched at each tieback location. In either case, the exposed face of the wall is unattractive.
Soil Nail Walls or Rock Nail Walls
Soil nailing is another method which is used to retain the ground adjacent to an excavated cut. In rock or rock-like ground formations, soil nailing may also be referred to as rock nailing. Soil or rock nailing is often preferred over the above-mentioned tied back and tied back element walls because soldier beams, timber lagging and numerous tiebacks are not required. This technique is thus less costly.
Soil nailing methods use untensioned tendons in grout-filled holes drilled into the ground behind a soil or rock cut. A soil nail system is shown in FIGS. 1 and 2. U.S. Pat. Nos. 3,638,435; 3,802,204 and Re. 28,977 are further exemplary of such soil nail systems.
As shown in FIGS. 1 and 2, in a soil nail wall, an array of nearly horizontal reinforcements, or soil nails 10, are installed in the soil mass as the excavation proceeds downwardly. A reinforced layer of pneumatically applied concrete 12 is used to support the exposed face of the cut between the soil nails 10. The pneumatically applied concrete 12 may be reinforced, for example, with a layer of welded wire reinforcing fabric 14.
During excavation, each lift is limited to a maximum depth at which the cut face is no longer self-supporting. A horizontal row of soil nails 10 is installed for additional support at this point. The soil nails 10 are essentially comprised of boreholes 8, grout 9 and nail tendons 10a. The boreholes 8 extend through the layer of pneumatically applied concrete 12 and into the ground adjacent the wall. The boreholes 8 are filled with grout 9, and nail tendons 10a are then installed in the holes 8 before the grout sets. One end of each nail tendon 10a extends outwardly from the skin to facilitate securing the tendon 10a to the outer wall, and each tendon 10a typically includes a bearing plate 10b and fastener 10c. Soil nails 10 and pneumatically applied concrete 12 are used to complete successively descending sections of the wall until the desired depth is reached as shown in FIG. 1. A concrete facing 16 is then applied. The concrete facing 16 is typically reinforced with horizontally and vertically disposed reinforcing steel 15. Also, prior art soil nail systems are often provided with drainage systems, such as drainage mats and weep holes.
In the prior art, the application of the concrete outer face 16 is accomplished using conventional means known to those of ordinary skill in the art. For example, pneumatically applied concrete may be used to form the entire outer face 16, or cast-in-place concrete may be formed and poured along the entire wall face 16. Each of these prior art methods suffers from certain drawbacks. The use of pneumatically applied concrete is disadvantageous because the face of the cut covered by the pneumatically applied concrete 12 is often irregularly shaped. This problem is particularly acute in rock or rock-like ground formations. Because a smooth, planar surface is not presented for pouring or blowing the outer face 16 against, additional concrete must be used to form the desired smooth, planar surface. Also, when cast-in-place concrete is used, the forming and pouring operations are highly labor intensive. Additional re-working of the cast-in-place concrete face is often required to repair imperfections in the formed surface caused by the placement of the forms or by improper pouring techniques.
A retaining wall system which uses rock nails is shown in FIGS. 3 and 4. This system is constructed in substantially the same manner as the soil nail systems described above, except it is constructed in a ground formation containing rock. Thus, rock nails 20 are used in cooperation with pneumatically applied concrete 22 instead of soil nails. Again, an outer face of cast-in-place concrete 26 is used to complete the retaining wall, resulting disadvantageously in a time- and labor-intensive process.
The above-described soil nail or rock nail systems utilize cast-in-place or pneumatically applied concrete to form the outer face. This is the most commonly used prior art configuration for soil nail or rock nail walls. However, it is known in the art to use prefabricated panels as the outer face. For example, U.S. Pat. No. 5,002,436 describes a retaining wall system which is comprised of a soil nail wall which is faced with precast concrete panels. The panels are connected to the exposed ends of the soil nails via an adjustable coupling means. Backfill, typically gravel or crushed rock, is then placed in the space between the exposed face of the cut and the panels. This configuration is disadvantageous in several respects. First, this method requires a sufficiently large distance between the layer of pneumatically applied concrete and the panel to allow for access by workers to make the mechanical connection from the nails to the panels. Second, the disclosed panel to soil nail connection significantly increases the overall cost of the retaining wall system. Finally, the panel to soil nail connection is embedded in backfill material and is thus prone to corrosion or deterioration from exposure to the environment.
This last disadvantage may be overcome somewhat by the prior art configuration shown in FIGS. 5 and 6. Rock nails or soil nails 30 and pneumatically applied concrete 32 are used to support the excavated cut as described above. When the desired depth of the excavation is reached, this system utilizes a facing comprised partially of cast-in-place concrete and partially of precast reinforced concrete. A plurality of precast concrete panels 36 are stacked alongside the wall at a distance from the pneumatically applied concrete facing. Each panel 36 is attached via a connecting means 36a, such as a strap and plate system, to a connecting rod (not shown) extending from the nail grout. As shown in FIG. 6, cast-in-place concrete 38 is used to complete the attachment of the precast concrete panels 36 to the rock nails or soil nails 30. The cast-in-place concrete 38 ties together the nails 30 along the cut face and the precast concrete panels 36 comprising the outer face. The exposed end 30a of the nail 30 may be bent upward, and extensive reinforcing steel 35 is employed in the area of the cast-in-place concrete 38. The remaining space between the precast concrete panels 36 and the pneumatically applied concrete layer 32 is filled with backfill material 39, such as gravel or crashed rock. In the prior art, the cast-in-place concrete 38 has been poured prior to the placement of the backfill material 39, with the cast-in-place concrete 38 and backfill 39 being separated by a crude bulkhead or form system. This bulkhead or form system has been constructed using plywood 37a and stakes 37b. The plywood 37a is individually cut and sized to span the gap between each respective facing element 36 and the layer of pneumatically applied concrete 32. A stake 37b is then drilled or driven into the ground, with the stake 37b positioned behind the plywood 37a to provide support.
While the system of FIGS. 5 and 6 provides protection from the environment for the connection of the panels 36 and the nails 30, it suffers from certain drawbacks. Initially, the disclosed cast-in-place concrete system necessitates time- and labor-intensive forming and pouring operations. First, the panels 36 must be supported sufficiently to resist the pressures exerted by the poured liquid concrete. Also, forms are required in the exposed area between the precast concrete panels 36. These forms are used to retain the cast-in-place concrete in place along its exposed face 38a. Because the liquid concrete exerts tremendous pressures on these forms when poured, the cast-in-place concrete must be placed in lifts. When the cast-in-place concrete is poured in this manner, seams in the concrete are formed between lifts. These seams present a discontinuous and unattractive outer surface along the exposed outer face 38a. Re-working of the exposed concrete face 38a will thus be required to remove imperfections in the finish and provide an acceptable appearance. In addition, since a portion of the ultimate outside facing is precast concrete 36 and a portion is cast-in-place concrete 38a, the color of the concrete will be dissimilar, requiring painting of all or a portion of the wall facing.
In addition, forms are also required at points between the precast concrete panels 36 and the pneumatically applied concrete 32 as shown in FIG. 6 at 37a. These forms 37a ensure that the poured liquid concrete 38 does not flow into areas which will later be filled with gravel or crushed rock backfill material 39. The forms 37a must be placed after erection of the precast concrete panels 36, which is often difficult because of the limited access to the space between the rear of the panels 36 and the layer of pneumatically applied concrete 32 disposed along the exposed cut face. Again, a bulkhead comprised of individually fitted plywood forms 37a and stakes 37b has been used in the prior art. However, this method has proven highly time- and labor-intensive because the distance from the back of the precast concrete panels 36 to the layer of pneumatically applied concrete 32 is not constant, but varies with the shape of the excavated cut face. Thus, each wood form 37a must be measured and cut to fit each individual panel 36 to face 32 connection. This process is time-consuming and tedious, and must be repeated at each column of nails along the face of the wall.
A further alternative prior art retaining wall system is shown in FIGS. 7 and 8. Once again, rock nails or soil nails 40 are used in conjunction with pneumatically applied concrete 42 in supporting the cut face while the excavation proceeds downwardly. Cast-in-place concrete 48 is also used to connect the precast concrete panels 46 to the rock nails or soil nails 40. In this regard, a connection means 43 is provided to attach the panels 46 to the cast-in-place concrete 48, which is itself attached to the nails 40. The connection means 43 may comprise anchors or loops. With this prior art construction, the entire outer face of the completed wall is comprised of precast concrete panels 46, and the entire space between the precast concrete panels 46 and the pneumatically applied concrete 42 skin is filled with cast-in-place concrete 48 and reinforced steel 45. No additional backfill material is used with this method. As noted above, since there is no reliable method for constructing the face of the pneumatically applied concrete 42 to close tolerances in the field, the space between the panels 46 and the skin 42 is typically variable, and is often quite large. Thus, extra reinforced cast-in-place concrete must undesirably be used to fill this space. This extra cast-in-place concrete serves no structural purpose, and merely leads to an increase in the expense of constructing the retaining wall. Additionally, the prior art does not provide a method for bracing the panels 46 during erection. Nor does the prior art provide for a method for supporting the panels 46 against the tremendous pressures exerted by the liquid concrete used in encasing the connection.
A still further alternative prior art retaining wall configuration is shown in FIGS. 9 and 10. This retaining wall system is similar to that described above with respect to FIGS. 7 and 8. Rock nails or soil nails 50 are again used in conjunction with pneumatically applied concrete 52 in supporting the cut face while the excavation proceeds downwardly. A connection means 53, such as anchor loops, is provided to facilitate the attachment of the precast concrete panels 56 to the rock nails or soil nails 50. Notably, as above, the prior art does not provide a method for bracing these panels 56 during erection, and a method for supporting the panels 56 against the tremendous pressures exerted by the liquid concrete used in encasing the connection is not provided. Nonetheless, with this prior art method, cast-in-place concrete 58, reinforcing steel 55 and the connection means 53 are used to connect the precast concrete panels 56 to the rock or soil nails 50. Additional backfill material 59, such as gravel or crushed rock, is used to fill the remainder of the open space between the panels 56 and the pneumatically applied concrete skin 52. However, the prior an fails entirely to disclose a form system or bulkhead 57 which would ensure that the poured concrete 58 does not flow into areas which will later be filled with the backfill material 59. Nor does the prior art teach a method for constructing such a form system or bulkhead 57 in the enclosed area behind the panels 56. Indeed, because the precast panels 56 are erected first, access to the area between the panels 56 and the layer of pneumatically applied concrete 52 is extremely limited. Placing detailed formwork in this area is thus difficult.
It can thus be seen that there is a need for an improved method of constructing a retaining wall with an outer face. In general, such a retaining wall system must be designed to overcome the disadvantages inherent in existing prior art retaining wall systems by providing a structurally sound retaining wall which is less costly to construct and which has an outer face which is ultimately pleasing to the eye. More particularly, there is a need for an effective retaining wall system which utilizes soil or rock nails in cooperation with outer facing elements.
Because soldier beams and tiebacks would not be required with such a wall, construction costs could be minimized. Further, unlike prior art walls with discontinuous and unattractive cast-in-place concrete outer faces, an outer face comprised entirely of facing elements would present an attractive surface which is ultimately pleasing to the eye.
In addition, careful application of any pneumatically applied concrete along the face of the cut would not be required since this layer of pneumatically applied concrete would be later covered by the facing elements. Elimination of the need for a smooth layer of pneumatically applied concrete is particularly attractive in rock or rock-like ground formations where the cut face is uneven and it is often difficult to apply such concrete evenly.
Further advantages could be attained by the existence of the space between the nail-supported excavated cut and the facing elements. An isolated cast-in-place concrete closure pour, bounded in part by the facing elements themselves, may be used in this space to further strengthen the wall and to encase the connection of the nails to the facing elements. The connection may thus be protected from corrosion caused by exposure to the environment. Also, if the facing elements and the excavated cut are separated, the remaining space between them may be filled with free draining backfill material. With such a configuration, an air space is created which prevents the ground formation from freezing and causing damage to the wall.